Well cementing is a process used in penetrating subterranean formations to recover subterranean resources such as gas, oil, minerals, and water. In well cementing, a well bore is drilled while a drilling fluid is circulated through the well bore. The circulation of the drilling fluid is then terminated, and a string of pipe, e.g., casing, is run in the well bore. The drilling fluid in the well bore is conditioned by circulating it downwardly through the interior of the pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the well bore. Next, primary cementing is typically performed whereby a slurry of cement in water is placed in the annulus and permitted to set, i.e., harden into a solid mass, to thereby attach the string of pipe to the walls of the well bore and seal the annulus. Subsequent secondary cementing operations, i.e., any cementing operation after the primary cementing operation, may also be performed. One example of a secondary cementing operation is squeeze cementing whereby a cement slurry is forced under pressure to areas of lost integrity in the annulus to seal off those areas.
Low density or lightweight cement compositions are commonly used in wells that extend through weak subterranean formations to reduce the hydrostatic pressure exerted by the cement column on the weak formation. Conventional lightweight cement compositions are made by adding more water to reduce the slurry density. Other materials such as bentonite and sodium metasilicate may be added to prevent the solids in the slurry from separating when the water is added as described in, for example, U.S. Pat. No. 4,370,166. Unfortunately, the addition of more water typically increases the cure time and reduces the strength of the resulting cement.
Lightweight cement compositions containing lightweight beads have been developed as a better alternative to the cement compositions containing large quantities of water. The lightweight beads reduce the density of the cement composition such that less water is required to form the cement composition. The curing time of the cement composition is therefore reduced. Further, the resulting cement has superior mechanical properties as compared to cement formed by adding more water. For example, the tensile and compressive strengths of the cement are greater.
Unfortunately, a problem often encountered when using lightweight beads to lower the density of a cement composition is that the beads tend to segregate within the dry composition, and in particular during transportation due to agitation. For example, the lightweight beads may segregate together in one area of the cement composition, leaving other areas with little or no lightweight beads. Such segregation may be caused by a variation in the densities of the lightweight beads and/or by the lightweight beads having a density different from that of the cement. This segregation is particularly likely to occur when a gas stream is used to fluidize the cement composition for loading or unloading into a transport vessel. It is also particularly likely to occur when the lightweight beads are cenospheres, i.e., hollow sphere primarily comprising silica (SiO2) and alumina (Al2O3). Cenospheres are a naturally occurring by-product of the burning process of a coal-fired power plant and thus have a wide range of sizes and densities. For example, cenospheres may vary in size from about 10 to about 350 micrometers (micron) and in specific gravity from about 0.3 to about 0.9.
Therefore, the density distribution of a dry cement composition containing lightweight beads typically becomes non-uniform during transport before the cement composition reaches its final destination, e.g., the site of a well bore. When a slurry is formed from the non-uniform cement composition and pumped into a well bore, the slurry most likely will not have a uniform density distribution. A cement column formed in the well bore using such a slurry also will most likely not have a uniform density distribution. As such, the design specifications of the density profile of the cement column will not be met. That is, the actual density profile of the cement column will be significantly different from the calculated density profile as determined by assuming the use of a homogenous composition. A need therefore exits to develop a way to eliminate the segregation of lightweight beads in a cement composition.